1. Field of the Invention
The present invention relates to the field of electrical water heater systems. More particularly, the present invention relates to the field of electronically controlled tankless water heater systems.
2. Description of the Prior Art
Generally, water heaters are well known in the art. These water heaters are utilized for a variety of residential and industrial purposes. The most common water heater system presently used is the conventional hot water heater tank system. The system pumps the water into a hot water holding tank and is heated to a relatively high temperature, for example, 140.degree. F. to 160.degree. F. One of the disadvantages with this application is that the temperature of the water being used would be less than the temperature at which the hot water tank is maintaining the water, then the water from a cold water supply line must be added to the hot water discharged from the hot water tank to reduce the temperature of the heated water to the desired temperature so the water can be useable. This results in a significant loss of energy in the form of heat dissipation from the hot water tank and additional heat dissipation to the environment through supply conduits between the hot water tank and the water outlet.
Another disadvantage is that the hot water heater tank system in most applications is located remote from the outlets. The hot water can take a long time to get to the outlet, plus the water temperature will vary until the hot water heats the pipe and arrives at the faucet.
In another prior art application, a tankless water heater provides significant improvements over conventional hot water holding tank systems. One improvement is the efficiency which the tankless water heater provides. Since energy in the form of heat is applied only when hot water is desired, the energy loss which would occur in a conventional hot water tank system is prevented.
The disadvantage with the presently used tankless water heaters is the stress produced on heating elements within the heat exchanger when full power is immediately applied to the heating elements as the heater is initially powered up. Full power is maintained during the total time the heater is used. This instantaneous application of full power can produce significant stress on the heating elements, and thereby shortening their life span.
Another disadvantage is that it is difficult to maintain a constant water temperature. If the flow rate varies the output water temperature will also vary. The output temperature of the tankless water heater depends upon the electrical capacity and the flow rate of the faucet or showerhead. At a given flow rate and electrical capacity, the temperature rise will be constant. Another problem occurs in the winter season, the output water temperature is too cold and in the summer season, the output water temperature is too hot. To compensate for these various factors, many manufacturers construct their tankless water heaters with a switch that changes the electrical capacity for summer and winter seasons or mix hot and cold water during the summer season, wasting water and energy.
Many times tankless water heaters also have to be installed with expensive anti-scald and pressure compensating valves which are not cost effective to the system.
Another disadvantage with prior art systems is that they utilize discrete and analog components to obtain the precise set-point temperature which can lead to inaccuracy of the desired temperature and cause overshooting or undershooting of the temperature. This method is slow to respond to varying inlet water temperature variations.
The following ten (10) prior art patents were uncovered in the pertinent field of the present invention.
1. U.S. Pat. No. 4,288,685 issued to Tommaso on Sep. 8, 1981 for "Flow-Activated Resistance Heater For Water" (hereafter "the Tommaso Patent").
2. U.S. Pat. No. 4,334,147 issued to Payne on Jun. 8, 1982 for "Power Control For Appliance Using High Inrush Current Element" (hereafter "the Payne Patent").
3. U.S. Pat. No. 4,337,388 issued to July on Jun. 29, 1982 for "Rapid-Response Water Heating And Delivery System" (hereafter "the July Patent").
4. U.S. Pat. No. 4,338,511 issued to Six on Jul. 6, 1982 for "Electronic Thermostat Equipped With An Energy-Saving Device" (hereafter "the Six Patent").
5. U.S. Pat. No. 4,595,825 issued to Gordbegli on Jun. 17, 1986 for "Thermostatically Controlled Electric Water Heater" (hereafter "the Gordbegli Patent").
6. U.S. Pat. No. 4,638,147 issued to Dytch et al. on Jan. 20, 1987 for "Microprocessor Controlled Through-Flow Electric Water Heater" (hereafter "the Dytch Patent").
7. U.S. Pat. No. 4,713,525 issued to Eastep on Dec. 15, 1987 for "Microcomputer Controlled Instant Electric Water Heating And Delivery System" (hereafter "the Eastep Patent").
8. U.S. Pat. No. 4,970,373 issued to Lutz eta. on Nov. 13, 1990 for "Electronic Temperature Control System For A Tankless Water" (hereafter "the Lutz Patent").
9. U.S. Pat. No. 5,058,804 issued to Yonekubo eta. on Oct. 22, 1991 for "Automatic Hot Water Supply Apparatus" (hereafter "the Yonekubo Patent").
10. U.S. Pat. No. 5,079,784 issued to Rist eta. on Jan. 14, 1992 for "Hydro-Massage Tub Control System" (hereafter "the Rist Patent").
The Tommaso Patent discloses a flow activated resistance heater for water. It includes a movable piston supported by a flexible membrane and provided with a small flow restricting opening separates an inlet chamber.
The Payne Patent discloses a power control system for an appliance using high inrush current element. It includes a microprocessor which has been designed by permanently configuring the read only memory (ROM) to implement the control scheme of the power control system.
The July Patent discloses a rapid response water heating and delivery system for quickly and accurately heating water to a selected set point temperature of the water. It includes a water vessel which houses electrical heating elements for heating the water as it flows through the vessel. A control system employs a derivative action which takes into account the speed at which the actual temperature of the water being discharged from the vessel changes with respect to the set point temperature and modifies the average power supplied to the heating elements to minimize the actual temperature will overshoot or undershoot the set point temperature. The derivative action functions by expanding and retracting the proportional band in a specific direction to control the amount of electrical power supplied to the heating elements. A three-term amplifier provides proportional, derivative and integral control to the thyritors. The derivative and integral functions are established by R-C circuit networks to change quickly to provide power to the heating element to change the temperature of the water in direction to expeditiously attain the set point value.
The Six Patent discloses an electronic thermostat equipped with an energy saving device.
The Gordbegli Patent discloses a thermostatically controlled electric water heater for heating a flow of pool or spa water.
The Dytch Patent discloses a microprocessor which controls a flow through electric water heater. It includes a plurality of heating elements each adapted to be switched on and off in response to the microprocessor whereby the heat dissipated to the flowing water from the electric heating elements can be varied by arranging for the elements to be switched on and off in different combinations.
The Eastep Patent discloses a microcomputer which controls an instant electric water heating and delivery system. The microcomputer operates in response to a user selected flow rate and water temperature inputs to calculate the temperature difference between the cold water input and the hot water delivery output from a multisection continuous flow electric water heater.
The Lutz Patent discloses a closed loop electronic temperature control system for a tankless water heater. It teaches a differentiator/integrator. The Lutz Patent teaches dual gain selection capability in providing closer and more precise regulation of the temperature of water output by the heat exchanger. By selecting a low gain when the error signal indicates a relatively large variance between the sensed and selected water temperatures, i.e., greater than .+-.3.degree. F., the overshoot by the system, which would likely result if only a high gain was used for the system, is substantially reduced. Switching to a high gain once the water temperature is within .+-.3.degree. F. of the selected temperature allows the system to react more quickly to changes in the sensed water temperature, thereby providing more precise regulation of the water temperature.
The Yonekubo Patent discloses an automatic hot water supply apparatus. It includes a gas-burned instantaneous water heater unit which is supplied with cold water from a cold water supply pipe, and discharges heated hot water through a hot water supply pipe. The water heater unit has a heat exchanger in which water from a water controller is heated by a burner. The temperature of the heated water is detected by a thermistor. The detected temperature is compared with a preset temperature by a controller which actuates a proportional gas control valve to control the burner so that water will be heated to a desired temperature.
The Rist Patent discloses a control system for a hydro-massage tub system. It includes a proportional control system for reducing power levels to a heater to maintain the temperature within .+-.2.degree. F. As the temperature falls below the set point temperature proportional power is applied to the heater to reheat water until it approaches the set point. The power is proportionally reduced as the temperature approaches the set point temperature so that overshoot or temperature, is eliminated preventing any overheating of the tub water. Any rise and fall of the temperature is applied to integrate circuit to control the power applied to the heater through the triac.
It is desirable to design a new tankless water heater system which improves the presently used tankless water heater through the application of a proportional, integrating and derivative calculation performed by a microprocessor. These calculation are used to determine the operating characteristics of the heating system and to control the heating system with minimum components.